Embodiments of the present invention are directed to radio frequency (RF) jamming. More particularly, embodiments of the present invention are directed to jamming RF switching devices used to trigger RF-triggered explosive devices (RTEDs).
Modern war-fighters are increasingly confronting attacks from RTEDs, particularly devices that use improvised triggering mechanisms. These “booby traps” are often triggered by RF switching devices found in ordinary household items such as garage-door openers and radio-controlled (RC) toy vehicles. RFTDs can be very effective because they are difficult to detect and to counter.
During the Gulf War, coalition forces encountered significant numbers of booby traps and improvised RTEDs. Most of these devices were located in numerous bunker complexes. By way of illustration, a common improvised RTED consists of booby-trapped 5-gallon cans of napalm actuated by a RF receiving device such as a RC toy actuator or a garage door opener. Another common booby trap encountered involved daisy chaining a group of Valmara 69 antipersonnel (AP) fragmentation mines, again actuated by a RF receiver.
The proficiency of attacks in war zones has increased in frequency with many of the attacks directed toward interdicting convoys.
In response to the use of these RTEDs, field commanders began trying to protect themselves by using RF transmitters, such as toy car remote controls, to attempt to pre-detonate RTEDs. While this worked in some cases, many devices like garage-door transmitters and car alarms use rolling codes or other “protected” RF transmissions that are difficult to predict or duplicate and do not operate at the same frequencies as the RC toy controllers. This greatly increased the number of types and frequencies of the RF jammers needed to attempt to protect a vehicle or convoy.
Typically, improvised RTEDs are remotely detonated using relatively simple, readily available low-technology devices, such as garage door openers, car alarms, Remote Keyless Entry (RKE) devices, door bells, RC toy car remotes, family radio service (FRS) and general mobile radio service (GMRS) two-way radios, cellular telephones, and pagers—all of which can be used to enable radio frequency command detonation (RFCD). Therefore, this implies that observation of the target area probably requires line-of-sight (LOS) observation points in many cases. However, the adaptation of using radios, cell phones, and other RC devices has given the enemy the standoff ability to watch forces from a distance and not be compromised.
The type of RF switching devices of concern are cheap, simple, small, low power, limited range (from 10 to 200 meters) and operate in various frequency bands from HF (3–30 MHz) through UHF (300–3000 MHz). Most of these devices operate with a simple On-Off-Keying (OOK) or Frequency-Shift-Keying (FSK) modulation. They typically initiate commands based on bit streams consisting of a series of pulses from the remote keying device. For most RF switching devices and RKEs, the receiver must receive two full consecutive correct bit streams, with each bit stream lasting for tens of milliseconds. If a bit in either bit stream is not received correctly, the receiving device will not activate.
The RF characteristics for several of these devices are presented below.
Remote Keyless Entry (US/EUROPE):                315 MHz –433 MHz,        1 mW Transmit Power,        25–100 kHz Bandwidth (BW),        Range to 100 m,        Superheterodyne Receiver (Local Oscillator) Sleep/Wake Mode (To Conserve Battery Power),        Receiver needs to receive 2 full bit streams to perform intended function.        
Garage Door Openers:                27 MHz–433 MHz,        1 mW Transmit Power,        25–100 kHz Bandwidth,        Range to 200 m,        Superheterodyne Receiver (Local Oscillator) on At All Times,        Receiver needs to receive 2 full bit streams to perform intended function.        
Cordless Phones:                800 MHz–5.8 GHz,        100 mW Transmit Power,        50 kHz–500 kHz Bandwidth,        Range to 200 m,        Superheterodyne Receiver (Local Oscillator) on at all times.        
The receivers of most RF switching devices and RKEs are superheterodyne receivers consisting of an antenna, an RF Filter, an RF amplifier, a mixer, a local oscillator, an intermediate frequency (IF) filter, and an IF amplifier. There are several ways to interfere with a superheterodyne receiver. Low-level interference sources on the same or adjacent channels can cause electromagnetic interference (EMI). Also high-level out-of-band signals can saturate the RF amplifier, causing desensitization, cross-modulation, or intermodulation products that cause EMI. These EMI interactions are antenna-induced products (i.e., the antenna is in the path from the interferer to the receiver). Another cause of EMI is high-level signals at the IF of the receiver which penetrate the body of the receiver. This interaction does not include the antenna and is commonly called “back-door” interference.
Large and expensive efforts have been undertaken by the military to address the RTED problem. However, all of the present solutions are very sophisticated, require large pieces of equipment, and are quite costly at over $500,000 per unit. A simpler, smaller, and cheaper solution would be highly desirable.
In other developments, Ultra-Wide Band (UWB) technology has also progressed in recent years. UWB technology has its origins in the development of time-domain (impulse response) techniques for the characterization of linear, time-invariant microwave structures. The advent of the time-domain sampling oscilloscope (Hewlett-Packard c. 1962) and the development of techniques for sub-nanosecond (baseband) pulse generation provided the requisite tools for further basic research. While there is no single definition of what constitutes a UWB transmitter, the Federal Communications Commission (FCC) uses the following definition for regulatory purposes:                An intentional radiator that, at any point in time, has a fractional bandwidth equal to or greater than 0.20 or has a UWB bandwidth equal to or greater than 500 MHz, regardless of the fractional bandwidth. 47 CFR 15.503 (d).        
In the early 1970's, impulse or baseband techniques were applied to a large number of potential applications ranging from low cost, high-resolution radar to specialized communications systems having low probability of detection (LPD) and low probability of interference (LPI). Within the United States, much of the early work in the UWB field (prior to 1994), particularly in the area of impulse communications, was performed under classified U.S. Government programs. Since 1994, much of the work has been carried out without classification restrictions, and the development of UWB technology has greatly accelerated. Recent UWB improvements have come about in the fields of communications, radar, and localization. Numerous manufacturers have begun producing UWB chips and the cost of UWB devices has decreased. UWB chip manufacturers include Motorola's newly spun-off chip unit, Freescale Semiconductor (XtremeSpectrum), Alereon, Staccato Communications, Wisair, FOCUS Enhancements, Inc., Jazz Semiconductor, Advanced Semiconductor Manufacturing Corporation, Hua Hong NEC Electronics Co., Ltd, and Intel Corporation.
A summary of some UWB applications, for both the military and commercial markets, is presented below:
Commercial Applications:                High Speed (20+Mb/s) local area networks (LANs) and wide area networks (WANs)        Altimeter/Obstacle Avoidance Radars (commercial aviation)        Collision Avoidance Sensors        RF Identification        Intelligent Transportation Systems        Intrusion Detection Radars a Precision Geolocation Systems        Industrial RF Monitoring SystemsMilitary/Government Applications:        Tactical Handheld & Network LPI/D Radios        Non-LOS LPI/D Groundwave Communications        LPI/D Altimeter/Obstacle Avoidance Radar        RF Identification        Intrusion Detection Radars        Precision Geolocation Systems        Unmanned Aerial Vehicle (UAV)/Unmanned Ground Vehicle (UGV) Datalinks        Proximity Fuses        LPI/D Wireless Intercom Systems        
The major advantages and disadvantages of UWB systems both result from the wide bandwidths associated with the ultra-short pulse waveforms that are used in most implementations of UWB technology. Although these ultra-short pulses result in the potential for high data rates for communicating and high-resolution imaging for radar applications, their associated wide bandwidths result in a potential for EMI over a wide range of frequencies. It is anticipated that UWB signals will be effective in interfering with the operation of these RF switching devices and RKEs because some portion of the UWB energy will be on-tune (thereby lowest power required to induce EMI) to the respective RF switching device and RKE receivers.
The RF characteristics for UWB devices are presented below:
Wideband Controlled Spectral Content (kHz To GHz)
High Peak Power (Effective Jamming) measured in units of Watts.
Low Average Power, measured in units of less than 1 mW.
Small Size, low power and low weight.
Although it is often regarded as new technology, the basic UWB technology has been around as long as wireless. Marconi's original spark transmission and all early wireless telegraphy were UWB. The military spent years investigating the application of UWB signals for high-resolution “carrier free” radar systems. Applications for UWB may be categorized as radar, location, and data communications.
UWB systems provide a potential for improved performance compared to legacy systems for certain military radio communication and sensing systems functions. However, the UWB systems also pose a potential threat to legacy systems because of potential EMI problems. The objective of the Defense Advanced Research Projects Agency (DARPA) Networking in Extreme Environments (NETEX) program was to create a wireless networking technology for the military user that enables robust connectivity in a wide spectrum of environments and support its integration into new and emerging sensor and communication systems. The NETEX effort investigated the susceptibility of selected military communication, navigation, and radar receivers to EMI from various UWB waveforms. The results of this investigation implied that UWB devices can easily cause EMI in legacy systems. The test also defined the UWB system parameters that caused the most effective EMI. The results of these tests demonstrated that UWB can be used effectively to interfere with simple RF switching devices and RKEs.
Seventeen selected military systems were tested to determine the susceptibility of legacy receivers to the very narrow pulses (and pulse trains) of transmitters associated with UWB systems. The selected military systems provided a representative sample of communications, navigation, and radar systems that are currently used in military applications. Typical parameters that influence receiver susceptibility are the sensitivity of the receiver, the levels of the desired and interfering signal sources, frequency and modulation of the desired signal source, pulse repetition frequency (PRF) of the UWB source, receiver bandwidth and operating frequency, and threshold levels associated with any responses.
The basic approach utilized during the NETEX testing was to subject each of the selected receivers to a number of “worst case” UWB waveforms and determine the conditions that cause EMI effects in the receiver. The results of these tests defined the receiver susceptibility threshold to these waveforms when the UWB emitter was connected directly to the receive antenna port (through a variable attenuator).
DARPA conducted over 1600 individual tests for different modes (39), at fixed frequencies (65) and during frequency hopping (5), and frequency ranges between 30 MHz and 16 GHz using 7 generic UWB waveforms. These UWB signal parameters were specified relative to selected victim RF and IFs. The UWB signal levels were not limited to FCC rules.
The tested generic UWB waveforms were as follows:                TW1—PRF is equal to, or a sub-harmonic of, the test frequency with no modulation or dithering of the PRF.        TW2—PRF is equal to, or a sub-harmonic of, the test frequency with the PRF waveform dithered +/−10%.        TW3—PRF is equal to the IF bandwidth of receiver under test with the PRF waveform dithered +/−1%.        TW4—PRF is equal to the IF bandwidth with PRF frequency modulated (FM)        TW5—PRF is equal to one tenth of the IF bandwidth with no modulation or dithering on PRF.        TW6—PRF is equal to ten times the IF bandwidth with no modulation or dithering on PRF.        TW7—PRF is equal to one hundredth of the IF bandwidth with no modulation or dithering on PRF.        
The tests performed measured receiver sensitivity, receiver susceptibility to white noise, receiver susceptibility to a small UWB signal (where the UWB is increased until it upsets the receiver and is then decreased until the receiver reacquires the desired signal), and receiver susceptibility to a large UWB signal (where the desired signal is increased until the receiver acquires it and then decreased until the signal reception at the receiver is upset).
The test results found that the EMI impact depended on the UWB power that fell within the receiver passband. As noted, these tests were performed to determine the susceptibility of various RF receiver (radars and communications systems) devices to UWB signals. The tests were not intended to evaluate the use of UWB signals for jamming purposes.
What would be useful would be a jamming system that would thwart the detonation of RTEDs using RF switching devices. Such jamming system would also be portable, use relatively small power sources, and be effective against a wide range of RF switching devices.